The present invention relates generally to phase change memory cells and a method for fabricating the same, and more specifically, to a phase change memory cell having a lower bottom electrode formed of a selectable material and a method for fabricating a self-aligned lower bottom electrode which does not require a lithographic masking process, is substantially seam free, and which has minimal topography (i.e., is flat).
A phase change memory cell requires a highly scaled portion of the memory cell to be defined. In order to define a highly controlled sub-lithographically sized aperture within the phase change memory cell, a keyhole transfer method is typically used. FIGS. 1A through 1D illustrate a typical keyhole transfer method. In FIG. 1A, a bottom electrode layer 10 and a memory cell layer 12 on top of the bottom electrode layer 10 are provided. A first dielectric layer 15 is formed on the bottom electrode layer 10 and an isolation layer 16 is formed on the first dielectric layer 15, and a second dielectric layer 17 is formed on the isolation layer 16. A photo resist layer 18 is formed over the second dielectric layer 17. A via 20 is formed to extend to the first dielectric layer 15 using the photo resist layer 18 via a lithography process. In FIG. 1B, the photo resist layer 18 is removed and the isolation layer 16 is recessed, creating overhang portions 17a and 17b of the second dielectric layer 17. In FIG. 1C, a conformal film 22 is deposited within the via 20 and pinched to form a void (i.e., a keyhole structure 24) in a lower region of the via 20. In FIG. 1D, the conformal film 22 is recessed and the keyhole structure 24 is transferred down into the first dielectric layer 15 to form a pore 26.
Currently, uniform keyhole formation and the ability to transfer the keyhole into an underlying layer or to a lower portion of the material which includes the keyhole structure, may depend on the underlying topography of the structure on top of which the keyhole structure is formed. Further, in the phase change memory cell, the electrode material which contacts the phase change material is required to be made of a specific material. The electrical and thermal resistivity of the electrode material as well as the affinity of the electrode material to chemically interact with the phase change material determines the suitability of the electrode material.